CONDENSED CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Treasury Stock	Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Increase (Decrease) in Stockholders' Equity
Cumulative impact from accounting change	$ (446)					$ (446)
Balance at beginning of period at Dec. 31, 2017	2,523,905	$ 47,555	$ (430,576)	$ 1,742,419	$ 2,077	1,162,430
Balance (in shares) at Dec. 31, 2017		475,547	14,936
Increase (Decrease) in Stockholders' Equity
Net income (loss)	117,231					117,231
Stock amortization and vesting	302	$ 53		249
Stock amortization and vesting (in shares)		534
Purchase of treasury stock	(207,135)		$ (207,135)
Purchase of treasury stock (in shares)			8,328
Cash dividends	(27,647)					(27,647)
Other comprehensive income	306				306
Balance at end of period at Mar. 31, 2018	2,406,516	$ 47,608	$ (637,711)	1,742,668	2,383	1,251,568
Balance (in shares) at Mar. 31, 2018		476,081	23,264
Balance at beginning of period at Dec. 31, 2018	2,088,159	$ 47,610	$ (1,334,688)	1,763,142	4,437	1,607,658
Balance (in shares) at Dec. 31, 2018		476,095	53,410
Increase (Decrease) in Stockholders' Equity
Net income (loss)	262,763					262,763
Stock amortization and vesting	(213)	$ 68		(281)
Stock amortization and vesting (in shares)		682
Purchase of treasury stock	(28)		$ (28)
Cash dividends	(29,605)					(29,605)
Other comprehensive income	(137)				(137)
Balance at end of period at Mar. 31, 2019	$ 2,320,939	$ 47,678	$ (1,334,716)	$ 1,762,861	$ 4,300	$ 1,840,816
Balance (in shares) at Mar. 31, 2019		476,777	53,410